An alternative is to take a piece of plastic cling wrap, float it on the surface of the paint/varnish or whatever that remains in your can and press the wrap against the in-side of the can above the surface of the material. If you really want to get fancy, stop by the supermarket and pick up a small piece of dry-ice. When you are done painting/varnishing or what have you, use the cling wrap trick, above followed up with a few chips of the dry ice dropped on the wrap. In a few minutes the ex-gasing of the dry ice will displace most of the air in the paint/varnish can with CO2 which is far less dangerous than your propane trick. BTDT

FWIW...

__________________"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."